This invention relates to a mercury-containing copper oxide superconductor having a high superconducting transition temperature.
Because superconductors have unique characteristics that (1) the electrical resistivity is zero, (2) they show perfect diamagnetism and (3) they show Josephson effect, superconductors are anticipated for utilization in various applications such as electric power transmission, electric generators, nuclear fusion plasma containment, magnetic levitation trains, magnetic shield and high speed computers.
An oxide superconductor (La1xe2x88x92xBax)2CuO4 having a high superconducting transition temperature Tc of about 30 K was found by Bendonorz and Muller in 1986. Since then, reports have been successively made on superconducting transition at high temperatures in connection with YBa2Cu3Ox (Tc=90 K) , Bi2Sr2Ca2Cu3Oy (Tc=110 K) Tl2Ba2Ca2Cu3Oz (Tc=125 K) and HgBa2Ca2Cu3Oz (Tc=135 K) A number of studies are now being made on the method of preparation, physical properties and application of these substances. In particular, mercury-containing copper oxide superconductors are considered to be promising because of their high superconducting transition temperature.
In order to practically use a copper oxide superconductor, it is important that the superconductor have a shape suitable for its utilization. For the utilization in electronic devices, for example, development of film forming techniques for superconductors is essential. Studies on the formation of mercury-containing copper oxide superconductor films have been extensively made all over the world since 1993 in which a mercury-containing copper oxide superconductor was first found.
A process for the preparation of a mercury-containing copper oxide superconductor film having Tc of 100K or more is known in which a precursor film composed of constituent elements is first prepared and then heat treated in the presence of mercury (Y. Q. Wang et al., Appl. Phys. Lett. 63 (1993) 3084).
A physical vacuum deposition method such as RF sputtering or pulsed laser ablation (PLD) is generally adopted for the production of the precursor film. When the precursor film is exposed to air, Ba contained therein reacts with moisture and carbon dioxide, which considerably adversely affect the formation of a desired mercury-containing copper oxide superconductor film. To prevent such a reaction, a mercury oxide film is generally provided over the precursor film (C. C. Tsuei et al., Science 263 (1994) 1259).
In the heat treatment, the precursor film is placed in a vacuum quartz tube together with pellets of Hgxe2x80x94Baxe2x80x94Caxe2x80x94Cuxe2x80x94O for the purpose of controlling the mercury vapor pressure. The heat treatment is performed at about 800xc2x0 C. under vacuum to obtain a mercury-containing copper oxide superconductor film having Tc of 130 K or more (W. N. Kang et al., Appl. Phys. Lett. 73 (1998) 381).
A mercury-containing copper oxide superconductor film having a critical current density Jc of about 106 A/cm2 or more at 77 K is also reported (S. L. Yan et al., Appl. Phys. Lett. 73 (1998) 2989).
As an application to an electronic device, a superconducting quantum interference device (SQUID) using a mercury-containing copper oxide superconductor film having a thickness of 2000 xc3x85 is reported (A. Tsukamoto et al., Appl. Phys. Lett. 73 (1988) 990). The junction is reported to have Jc of 4xc3x97103 to 6xc3x97104 A/cm at 77 K and IcRn of 450 xcexcV which is greater than IcRn values of currently most widely studied Y123 type superconductors (200 to 300 xcexcV). Further, the SQUID is reported as operable at 111 K. These characteristics suggest high potential of mercury-containing copper oxide superconductors.
A mercury-containing copper oxide superconductor is a layer compound. The crystal grain has a plate-like shape as illustrated in FIG. 3. A desired superconductor film is composed of c-axis oriented plate-like crystals 11A continuously arranged on a substrate 10. However, it has been found that known mercury-containing copper oxide superconductor film has a number of defects. Thus, as shown in FIGS. 4(a)-4(c), there are steps 13, holes 14 and gaps 15 in known films. The defects also include outgrowths whose crystal orientations are not uniform (see FIG. 6). These defects cause a reduction of Jc as well as variation of superconductive characteristics at different locations of the film and interfere the fabrication of electronic devices.
It is an object of the present invention to provide a mercury-containing copper oxide superconductor film free of the above-described defects.
Another object of the present invention is to provide a superconductor film of the above-mentioned type which has a desired thickness.
It is a further object of the present invention is to provide a superconductor film of the above-mentioned type which has two or more mercury-containing copper oxide superconductor layers having different compositions so that the superconductive characteristics of the film is modulated.
It is a further object of the present invention to provide a method which can prepare the above mercury-containing copper oxide superconductor film.
In accomplishing the foregoing object, the present invention provides a superconductor having a substrate, and a superconducting film provided on the substrate and having a thickness in the range of between 300 xc3x85 to 950 xc3x85, wherein the superconducting film contains Hg, M which is Ba, Sr or Ca, Cu and O.
The present invention also provides a method of producing a superconductor, which includes the steps of:
(a) forming, on a substrate, a precursor laminate film having a first film containing M which is Ba, Sr or Ca, Cu and O and a second, mercury oxide film provided on the first film;
(b) placing the precursor laminate film-bearing substrate in a closed vacuum chamber together with a first pellet containing Hg, M which is as defined above, Cu and O and a second pellet containing M which is as defined above, Cu and O; and
(c) heating the precursor laminate film-bearing substrate, the first and second pellets in the vacuum chamber to form, on the substrate, a superconducting film having a thickness in the range of between 300 xc3x85 to 950 xc3x85 and containing Hg, M, Cu and O.
The mechanism by which the known mercury-containing copper oxide superconductor film is formed is considered to be as follows.
As shown in FIG. 5(a), a substrate 10 on which a precursor film consisting of a Mxe2x80x94Cuxe2x80x94O layer and a Hgxe2x80x94O layer is provided is placed in a vacuum chamber together with pellets. Then, the contents in the chamber are heated so that a mercury-containing copper oxide superconductor film having a thickness of 1000 xc3x85 or more is formed on the substrate. As shown in FIG. 5(b), a liquid phase 12 composed of Hg, M, Cu and O is formed during the heat treatment. Seed crystals are formed in the liquid phase 12 and grow gradually. In the region adjacent to the surface of the substrate 10 where the surface tension of the liquid phase 12 effectively acts on the growing crystal grains, the c-axis thereof orient in the direction normal to the substrate. However, crystal grains 11 A growing in the region remote from the surface of the substrate 10 orient at random. As a consequence, as shown in FIG. 6, there are formed steps 13, holes 14, gaps 15 and outgrowths 16.
In contrast, the superconductor film according to the present invention is considered to be produced as follows.
As shown in FIG. 1(a), a substrate 10 on which a precursor film 11B having a thickness smaller than the above and consisting of a Mxe2x80x94Cuxe2x80x94O layer and a Hgxe2x80x94O layer is provided is used. The precursor film-bearing substrate is placed in a vacuum chamber together with pellets. Then, the contents in the chamber are heated. As shown in FIG. 1(b), a liquid phase 12 composed of Hg, M, Cu and O is formed during the heat treatment. Seed crystals are formed in the liquid phase 12 and grow gradually. Since a the precursor film 11B is thin, most of crystal grains grow in the region adjacent to the surface of the substrate 10. Thus, the orientation of the crystal grains receives a strong influence of the crystal orientation of the substrate 10. Accordingly, the c-axis thereof orient in the direction normal to the surface of the substrate, with the a-axis thereof being oriented in the direction corresponding to the crystal orientation of the substrate 10. Moreover, since the liquid phase 12 can effectively wet both the surface of substrate and the crystal grains in contact therewith, the crystal grains grow in the lateral direction in an enhanced manner. Thus, as shown in FIG. 1(c), the mercury-containing copper oxide superconductor film 11 is free of steps, holes, gaps and outgrowths and is flat and uniform. Further, the crystal grains are tightly connected with each other.